By Larry Carley
Technical Editor

Filters are the first line of defense against contaminants, whether it’s airborne contaminants entering the engine, wear particles, dirt or soot contaminants inside the engine, wear or dirt particles inside the transmission or dirt or rust particles in the fuel. And now that so many vehicles also have cabin air filters, unwanted contaminants also include dirt, soot, odors and even bacteria entering the passenger compartment.

AIR FILTERS
Most air filters are made of resin-treated cellulose that is combined with synthetic fibers. The typical paper air filter will trap most dirt particles that are five to six microns in size and larger, along with 80 to 90 percent of the particles down to a couple microns in size. Some of the best premium air filters will stop anything two microns or larger in size and most particles from one to four microns in size.

Air filters have to be efficient at trapping dirt, but also unrestrictive so the engine can breathe normally. For every gallon of fuel burned, 10,000 to 12,000 gallons of air will be pulled through the air filter. That’s a lot of air and it doesn’t take long for dirt to build up in the filter, especially if the vehicle is being driven in a dirty environment or on dusty gravel roads.

If the air filter is not changed, the trapped dirt will close up the pores in the filter and restrict airflow. The filter’s efficiency to trap dirt will actually go up the dirtier it gets. But at the same time, the buildup of dirt will also make it harder and harder for air to pass through the filter. Consequently, the engine has to suck harder to get the same volume of air. This results in a noticeable drop in throttle response, engine performance and fuel economy. It can also cause carbon monoxide emissions to go up and may even cause the vehicle to fail a tailpipe emissions test. The OEM recommended replacement interval for many air filters today ranges from 30,000 to 60,000 miles, but these intervals are for ideal operating conditions. A more realistic service interval is to inspect the filter yearly and replace it every two years or 24,000 miles, or as needed depending on driving conditions.

CABIN AIR FILTERS
Tests have shown that the concentration of pollutants that occur inside a vehicle while it is being driven in heavy traffic is often significantly higher (up to six times higher) than the level of pollutants outside the vehicle. This occurs because exhaust fumes from the vehicles in front of you are drawn in through your HVAC system (unless it is operating in the “Recirc” mode). The cabin air filter helps reduce the “exhaust tunnel effect” to improve air quality inside the vehicle.

Another benefit of a cabin air filter is that it keeps the A/C evaporator clean. This helps maintain peak cooling efficiency and reduces musty A/C odors caused by the growth of microbes on the surface of the evaporator. Many cabin air filters also contain a layer of activated charcoal to trap annoying odors and harmful pollutants. These are called “combination” filters.

As a general rule, “dust only” cabin air filters should be inspected yearly, and replaced every 20,000 to 30,000 miles or more often as needed. For the combination cabin air filters with the odor-absorbing layer of activated charcoal, yearly replacement is usually required to maximize odor control.

OIL FILTERS
A buildup of contaminants inside an oil filter can restrict oil flow to the engine. Like air filters, the dirtier the oil filter gets, the more efficient it is at trapping contaminants. It also slows down the flow of oil through the filter. Eventually, this will cause the spring-loaded oil filter bypass valve to open so the engine will continue to receive oil pressure. But the oil bypassing the filter will have no filtration whatsoever, so any contaminants that are in the crankcase will circulate throughout the engine. This can cause accelerated engine wear and premature engine failure.

The oil filter should be changed every time the oil is changed. Some people may try to economize by changing the oil filter every other oil change, which is probably okay as long as: they are changing their oil every 3,000 miles; the engine does not have a lot of miles on it; there is minimal blowby; and they are not driving on dusty gravel roads. The price of a filter is a lot less than the price of an overhaul, so don’t start out a fresh oil change with a pint to a quart of old dirty oil in the filter.

TRANSMISSION FILTERS
The transmission filter’s job is to keep the transmission fluid clean to minimize wear and the risk of control solenoids jamming or sticking. On most vehicles, the filter is located inside the pan that covers the bottom of the transmission. Consequently, the filter is out of sight and out of mind to many motorists. In fact, many motorists may not even realize their transmission has an internal filter.

On most late-model vehicles, there is no recommended replacement interval for the transmission filter. Or, it may be 100,000 miles. Even so, most transmission experts will tell you that changing the transmission fluid and filter every 50,000 miles can significantly reduce the risk of transmission-related problems and premature failure.

Replacing the filter requires dropping the transmission pan, so your customer will also need a new pan gasket when changing the filter.

FUEL FILTERS
Here’s another filter that has disappeared from the consciousness of many motorists today. Like transmission filters, the fuel filter is often tucked away in a hard-to-find location somewhere under the vehicle or in the back of the engine compartment. Many owner’s manuals no longer list a recommended replacement interval for this filter and some say it’s a “lifetime” filter, which means you don’t have to change it.

The fuel filter is typically located somewhere in the fuel line between the fuel pump and the fuel injection supply rail on the engine. On some late-model Chrysler and Ford vehicles, the filter is located inside the fuel tank and is part of the fuel pump assembly.

The fuel filter’s purpose is to trap dirt and rust particles before they reach the injectors or the fuel pressure regulator. There is also a screen or sock on the fuel pump inlet, but this is only to prevent large chunks of debris (70 to 100 microns in size and larger) in the fuel tank from being sucked into the fuel pump. Most fuel filters will trap particles three microns or larger in size and some will trap particles that are even smaller. And just like air and oil filters, filtering efficiency goes up the dirtier the filter gets. But at the same time, so does the resistance to flow.

A plugged fuel filter is bad news because it causes a drop in fuel flow that chokes off the engine’s fuel supply. A dirty filter may flow enough fuel at idle and low rpm not to cause any noticeable problems, but it may not pass enough fuel at higher engine speeds causing a loss of power and hesitation. If the filter plugs up, it can cause the engine to stall or make it hard to start. Many fuel pumps have been replaced unnecessarily because the real problem was a plugged fuel filter, not a bad pump.

Many experts recommend replacing the fuel filter for preventive maintenance every two to three years or 30,000 miles, or anytime a fuel restriction is suspected. The fuel filter should also be replaced if the fuel pump has failed and is being replaced.